Manoj S. Nair, Ph.D
Postodoctoral Fellow, Biochemistry
812 Biosciences bldg, 484 w. 12th ave
Columbus, OH 43210
Nair.30@osu.edu

1.
2.
3.



Types of transport across membranes
Passive transport
Carrier mediated
Active transport
Nernst equilibrium for ion transport
Mechanism of ion transport (K-selectivity
filter)
Endocytosis of proteins/protein domains
Introduction to Cellular Biophysics
A. Molecular Basis of Membrane Transport.
Essential Cell Biology Alberts, Bray, et al.



Transport up a
concentration
gradient
Uses energy (ATP)
May also cause
charge gradient
across the membrane
causing the molecule
to move against the
membrane potential.
Properties of “Active” membrane pumps
Example of a Na+/K+ pump ATPases (sometimes GTPases)
Na+/K+ pump
uses  30%
resting ATP
Active Pumps are used to transport materials against their electrochemical
gradient
Essential Cell Biology Alberts, Bray, et al
a) Uniport: 1 type of solute is
transported
Eg: Valinomycin (K+ transport)
V alinom ycin
H 3C
N
CH3
CH
O
CH
C
O
O
H
C
CH
H3C
L -valine
H
C
O
N
H
C
H
CH
CH 3 H3C
D -hydroxy-
isovaleric acid
C
CH3 O
O
CH
3
CH3
D -valine
C
L -lactic
acid
Valinomycin is a carrier for K+.
It is a circular molecule, made up of 3 repeats
of the sequence shown above.
Puckering of the ring, stabilized
by H-bonds, allows valinomycin V a lin o m y cin
to closely surround a single
O
+
unhydrated K ion.
O
O
+
Six oxygen atoms of the
K
ionophore interact with the
O
O
+
bound K , replacing O atoms of
O
waters of hydration.
H yd rop h ob ic
Valinomycin is highly selective for K+ relative to Na+.
The smaller Na+ ion cannot simultaneously interact with
all 6 oxygen atoms within valinomycin.
Thus it is energetically less favorable for Na+ to shed its
waters of hydration to form a complex with valinomycin.
V a lin o m y cin
O
O
K
O
O
+
O
O
H yd rop h ob ic
Whereas the interior of the valinomycin-K+ complex is
polar, the surface of the complex is hydrophobic.
This allows valinomycin to enter the lipid core of the
bilayer, to solubilize K+ within this hydrophobic milieu.
Crystal structure
V al-K
K
+
V al-K
+
+
K
V al
+
V al
m em brane
Valinomycin is a passive carrier for K+. It can bind or
release K+ when it encounters the membrane surface.
Valinomycin can catalyze net K+ transport because it
can translocate either in the complexed or
uncomplexed state.
The direction of net flux depends on the
electrochemical K+ gradient.
b) Symport: 2 different solutes transported together in one direction
Eg: Glucose –Na+ tranporter in epithelial cells
Lactose permease: H+ -lactose symport
c)Antiport: 2 different solutes transport in opposite directions
Eg: Adenine nucleotide translocase (ATP/ADP exchanger)
Properties of
transmembrane  helices:
Amphiphilic nature
Designer Peptides of
Ser & Leu:
Formed a hexamer
channel in phospholipid
membranes.
S.R.Goodman. 1998
What is the mechanism for ion selectivity of channels?
This is a frontier of biophysics.
With Passive Channels, ions or other substances move
DOWN their electrochemical gradient
+ +
-
-
-
+
+
+
-
Basic structure of the potassium channel.
Doyle et al. Science, 1998. Nobel Prize in Chemistry in 2003
KcsA Selectivity Filter
• “Gated” channels i.e. channels that open in response to
physiological stimuli
Receptor-activated gate
Essential Cell Biology Alberts, Bray, et al.